Digital amplification device

ABSTRACT

A digital amplification device has a digital amplifying unit, a low-pass filter for integrating a digital signal, and a current feedback circuit that is transformer-coupled to the low-pass filter and feeds back current to the differential input terminal. A signal related to distortion is fed back and the distortion is reduced.

TECHNICAL FIELD

The present invention relates to reduction of distortion of a digitalamplification device, especially to a digital amplification device usedin a car audio equipment.

BACKGROUND ART

A conventional digital amplification device designed in consideration ofreduction of coil distortion of a low-pass filter is disclosed inJapanese Laid-open Patent No. 2000-307359.

FIG. 6 is a circuit block diagram of the conventional digitalamplification device. ΔΣ modulator (ΔΣ in FIG. 6) 101 converts analoginput signal 105 into a pulse density modulation signal.Constant-voltage switch (SW in FIG. 6) 102 is formed of a switch elementhaving a small on-resistance such as a metal-oxide semiconductorfield-effect transistor (MOSFET), and power-amplifies an output signalof ΔΣ modulator 101. Low-pass filter (LPF in FIG. 6) 103 is formed of acoil and a capacitor. Cut-off frequency of low-pass filter 103 is setlower than the switching frequency of constant-voltage switch 102. Theswitching component included in an output signal of constant-voltageswitch 102 is attenuated, and a power output of constant-voltage switch102 is smoothed and demodulated to an analog output signal. Feedbackcircuit (β in FIG. 6) 104 feeds back an output of low-pass filter 103 toan input of ΔΣ modulator 101. In this configuration, a distortionoccurring in low-pass filter 103 is detected based on the outputvoltage, and is fed back as a negative phase component to the input partto improve the distortion factor.

Distortion of a coil used in the low-pass filter also causes a problemabout sound quality. The digital amplification device requires alow-pass filter for integrating pulse signals, but especially coildistortion at a high magnitude output is a large factor of degradationof the sound quality. This distortion is caused by a nonlinearcharacteristic of a B-H curve of a magnetic material used for the coil,and is current distortion generated by current. The larger the currentis, the greater the effect of the nonlinear characteristic is.

Especially, a car audio equipment is often operated by a 12V system as apower source for an automobile, so that a high power output must beobtained by low power source voltage. For this purpose, a method ofdecreasing impedance of a speaker as a load is used. When the impedanceof the load is decreased, however, current of the load increases andhence the distortion of the coil of the low-pass filter in the digitalamplification device further increases to degrade the sound quality.

For decreasing the distortion factor of low-pass filter 103 in thedigital amplification device, a means for detecting output current isuseful. When only a resistor works as a load, the output voltage isproportional to the output current, and therefore the distortion of thecoil can be accurately reduced by detecting the output voltage. When anonlinear load such as a speaker is used, however, the output currentcan largely distorts even if the output voltage does not distort. It isdifficult to correct this distortion by detecting the output voltage.

For increasing the output efficiency, an on-resistance of a switchingtransistor and a resistance component of an output circuit must beminimized. Here, the switching transistor constitutes the output part ofa digital amplifying stage, and the resistance component is, forexample, a resistance component of the coil in low-pass filter 3.Therefore, it is not preferable to dispose a resistor or the like in anoutput circuit in series.

DISCLOSURE OF THE INVENTION

A digital amplification device has the following elements:

-   -   a digital amplifying unit including a differential input        terminal for inputting an analog signal and a differential        output terminal for outputting a digital signal, converting the        analog signal to the digital signal, amplifying the power, and        outputting the digital signal;    -   a low-pass filter for integrating the digital signal; and    -   a load output terminal for outputting a signal from the low-pass        filter,

Wherein the digital amplification device further comprises a currentfeedback circuit for feeding back, to the differential input terminal, asignal related to a signal provided by transformer coupling with thelow-pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram of a digital amplification device inaccordance with exemplary embodiment 1 of the present invention.

FIG. 2 is a transfer characteristic diagram of a first current feedbackcircuit of the digital amplification device in accordance with exemplaryembodiment 1.

FIG. 3 is a characteristic diagram of total harmonic distortion foroutput power of the digital amplification device in accordance withexemplary embodiment 1.

FIG. 4 is a schematic diagram of a first transformer coupling type coilof a digital amplification device in accordance with exemplaryembodiment 2 of the present invention.

FIG. 5 is a circuit block diagram of a digital amplification device inaccordance with exemplary embodiment 3 of the present invention.

FIG. 6 is a circuit block diagram of a conventional digitalamplification device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides digital amplification devices foraddressing the problems of the conventional digital amplificationdevice.

The digital amplification devices in accordance with exemplaryembodiments of the present invention are described with reference to thedrawings.

Exemplary Embodiment 1

FIG. 1 is a circuit block diagram of a digital amplification device inaccordance with exemplary embodiment 1 of the present invention.

Analog input terminal 11 is connected to differential input terminal 20of digital amplifying unit 13 through input impedance element 12 (Z inFIG. 1). Differential output terminal 21 of digital amplifying unit 13is connected to load output terminal 22 through low-pass filter 14.Low-pass filter 14 has first transformer-coupling-type coil 15,capacitor 16 interposed between first transformer coupling type coil 15and load output terminal 22, and bypass capacitor 17 interposed betweenload output terminal 22 and the ground. A primary side coil of firsttransformer coupling type coil 15 is interposed between differentialoutput terminal 21 and load output terminal 22 in series. One end of asecondary coil of first transformer coupling type coil 15 is grounded,and the other end is connected to differential input terminal 20 throughfirst current feedback circuit (βC in FIG. 1) 18. Differential inputterminal 20 is also connected to differential output terminal 21 throughvoltage feedback circuit (βV in FIG. 1) 19.

Operations of the digital amplification device are describedhereinafter.

An analog audio input signal given to analog input terminal 11 is fed todifferential input terminal 20 of digital amplifying unit 13 through apair of input impedance elements 12. This analog signal is converted toa digital pulse width modulation (PWM) signal having a frequencysufficiently higher than that of the analog audio signal and amplifiedin power by digital amplifying unit 13. The amplified PWM signal is fedfrom differential output terminal 21 to low-pass filter 14. The PWMsignal is integrated by low-pass filter 14, is supplied to speaker 33through load output terminal 22, and is reproduced as a sound. Voltageof the digital signal supplied from differential output terminal 21 ofdigital amplifying unit 13 is fed back to differential input terminal 20through voltage feedback circuit 19. Current of the digital signal isfed back from the secondary coil of first transformer coupling type coil15 to differential input terminal 20 through first current feedbackcircuit 18. Here, the first transformer coupling type coil 15 is acomponent of low-pass filter 14.

FIG. 2 is a transfer characteristic diagram of first current feedbackcircuit 18. In FIG. 2, curve 23 shows a voltage characteristic inducedin the secondary coil of first transformer coupling type coil 15 havinga gradient of coupling coefficient M, and curve 24 shows a transfercharacteristic of first current feedback circuit 18.

In first current feedback circuit 18, a switching frequency component asdistortion must be attenuated by making cut-off frequency Fc of firstcurrent feedback circuit 18 lower than switching frequency Fs of the PWMsignal. The loop gain and phase of first current feedback circuit 18must be adjusted to cancel a distortion component of signal frequencyFt. In other words, third harmonic distortion occurring in firsttransformer coupling type coil 15 is largest in the digitalamplification device. When the transfer characteristic of first currentfeedback circuit 18 is adjusted so that only third harmonic distortionis fed back, the third harmonic distortion can be largely reduced. Inthis case, only an error signal as the third harmonic distortion is fedback, so that the feedback gain may be simply inverse number of gain Aof digital amplifying unit 13, namely 1/A. Thus, the feedback amount offirst current feedback circuit 18 is adjusted to be the largest at afrequency three times higher than signal frequency Ft, namely 3×Ft,thereby efficiently reducing the third harmonic distortion of signalfrequency Ft.

FIG. 3 is a characteristic diagram of the total harmonic distortionfactor for output power of the digital amplification device inaccordance with exemplary embodiment 1. The horizontal axis shows theoutput power, and the vertical axis shows the total harmonic distortionfactor (THD+N in FIG. 3). Polygonal line 25 shows a characteristic whenfirst current feedback circuit 18 is not disposed, and polygonal line 26shows a characteristic when first current feedback circuit 18 isdisposed. When first current feedback circuit 18 with first transformercoupling type coil 15 is disposed, about 20 dB or higher of improvementis recognized at output power of 1 W to 30 W comparing with theconventional digital amplification device.

FIG. 3 shows a characteristic example provided when frequency F of theanalog input signal is 50 Hz, output load RL of the digitalamplification device is 1 Ω, power source voltage Vcc used for drivingthe digital amplification device is 14.4 V.

The structure discussed above can reduce distortion of the low-passfilter with the current feedback circuit that detects and feeds back thecurrent distortion causing the distortion of the low-pass filter.

Exemplary Embodiment 2

FIG. 4 is a schematic diagram of first transformer coupling type coil 15of a digital amplification device in accordance with exemplaryembodiment 2 of the present invention. FIG. 4 shows the firsttransformer coupling type coil formed of a toroidal double winding. Theresistance component of first transformer coupling type coil 15 causes aloss and hence must be decreased. The resistance component isproportional to the number of turns of the coil winding, so that it isrequired to provide a desired inductance with small number of turns. Itis therefore preferable to employ a core material having high magneticpermeability.

However, a material having high magnetic permeability has generally aB-H curve with high nonlinearity. The object of the present embodimentis to compensate the nonlinearity of the B-H curve, but it is notpreferable that the B-H curve has extreme nonlinearity. Toroidal core 27having a B-H curve with good linearity is used.

The number of turns of primary winding 28 is set so as to provide theinductance of low-pass filter 14. For decreasing the resistancecomponent, winding as thick as possible is preferably used if apredetermined number of turns can be obtained.

Secondary winding 29 may be thin. The number of turns of secondarywinding 29 can be determined substantially arbitrarily keeping a balancebetween the number and the transfer characteristic of first currentfeedback circuit 18. However, the number of turns is preferably a few ormore so as to stabilize coupling coefficient M. Fixing the winding withan adhesive also stabilizes coupling coefficient M.

A toroidal coil often used in the conventional digital amplificationdevice is employed in the structure discussed above, so that a printedboard pattern substantially similar to the conventional art can be used.Additionally, a pattern taking measures against radiation noise can beemployed and the structure of the conventional digital amplificationdevice can be used with littile change, so that current can be detectedat low cost.

Exemplary Embodiment 3

FIG. 5 is a circuit block diagram of a digital amplification device inaccordance with exemplary embodiment 3 of the present invention. In FIG.5, part of the digital amplification device different from that ofexemplary embodiment 1 is mainly described hereinafter.

Second transformer-coupling-type coil 30 is one component of a linefilter of a power source input part, the primary coil constitutes aninductance component of the line filter, and the resistance component ispreferably small. The secondary coil electromagnetically connected tothe primary coil is formed of two coils, these coils have substantiallythe same number of turns and reverse polarization, and one end of eachcoil is grounded.

Capacitor 31 forms a capacitance component of the line filter, and isconnected to the output side of the primary coil of second transformercoupling type coil 30. For supplying current having small fluctuation todigital amplifying unit 13, it is preferable to set capacity ofcapacitor 31 to be sufficiently large.

An external power is given to power-source input terminal 34, is passedthrough the primary coil of second transformer coupling type coil 30,and is supplied as power-source 35 to digital amplifying unit 13.Current is thus supplied to digital amplifying unit 13.

A pair of second current feedback circuits (β1 in FIG. 5) 32 areconnected to respective non-grounded terminals of the secondary coil ofsecond transformer coupling type coil 30. Polarity and gain of secondcurrent feedback circuits 32 are set to cancel ripple voltage of thepower source circuit. Only for adjusting the gain, second currentfeedback circuits 32 can be formed of only a resistor. However,switching waveform is generally superimposed on the power source, sothat the second current feedback circuits 32 may be formed of low-passfilters having cut-off frequency not lower than a reproduced audiofrequency band.

Operations of the digital amplification device are describedhereinafter.

A power source line supplied from power-source input terminal 34 todigital amplifying unit 13 has small yet some impedance, so that ripplecurrent occurs in the power source line. This power source provides areference voltage of digital amplifying unit 13, so that the ripplecurrent becomes a distortion component of the output as it is.Therefore, second transformer coupling type coil 30 detects currentdistortion, assumes it as ripple current of the power source, and addsit to the input signal. At this time, second current feedback circuits32 adjust a correction amount, and therefore the distortion componentcaused by the ripple current of the power source can be cancelled in theoutput voltage from digital amplifying unit 13.

When impedance such as a resistor is disposed in the power source line,the impedance causes a loss to decrease efficiency. However, detectingthe distortion current with second transformer coupling type coil 30 canrealize the cancellation of the distortion component without increasingthe resistor component.

In the structure described above, by detecting the distortion currentwith second transformer coupling type coil 30, the ripple current of thepower source can be detected as the distortion current withoutincreasing the impedance of the power source line. Feeding back thedistortion current can reduce the distortion caused by the impedance ofthe power source line.

As discussed in embodiment 1 to embodiment 3, the digital amplificationdevice of the present invention has the first current feedback circuitthat detects and feeds back the current distortion causing thedistortion in the low-pass filter, so that the distortion of thelow-pass filter can be reduced.

The digital amplification device of the present invention also has thevoltage feedback circuit for feeding back voltage of a digital signalfed from the differential output terminal to the differential inputterminal, so that both current and voltage can be fed back and thedistortion of the low-pass filter can be further reduced.

The digital amplification device of the present invention has a low-passfilter formed of a toroidal double winding and a capacitor, so that itcan go into actual use with little change from the structure of theconventional digital amplification device. The current can therefore bedetected at low cost, and the distortion can be reduced at low cost.

The digital amplification device of the present invention can reducedistortion of the low-pass filter with the first current feedbackcircuit that detects and feeds back current distortion causing thedistortion in the low-pass filter. By the second current feedbackcircuit, the digital amplification device can also reduce distortioncaused by the impedance of the power source circuit.

Thus, the digital amplification device of the present invention has acurrent feedback circuit that is transformer-coupled to the low-passfilter and feeds back current to the differential input terminal. Thecurrent feedback circuit detects and feeds back the current distortioncausing distortion of the low-pass filter, thereby reducing thedistortion of the low-pass filter.

Industrial Applicability

A digital amplification device of the present invention can reducedistortion of a low-pass filter by current feedback. The digitalamplification device of the present invention can also reduce distortioncaused by the impedance of the power source circuit by current feedbackfrom a power source circuit.

Reference Numerals

-   11 Analog input terminal-   12 Input impedance element-   13 Digital amplifying unit-   14 Low-pass filter-   15 First transformer coupling type coil-   16 Capacitor-   17 Bypass capacitor-   18 First current feedback circuit-   19 Voltage feedback circuit-   20 Differential input terminal-   21 Differential output terminal-   22 Load output terminal-   23 Voltage induced in secondary coil-   24 Transfer characteristic of current feedback circuit-   25 Characteristic when current feedback circuit is not disposed-   26 Characteristic when current feedback circuit is disposed-   27 Toroidal core-   28 Primary winding-   29 Secondary winding-   30 Second transformer coupling type coil-   31 Capacitor-   32 Second current feedback circuit-   33 Speaker-   34 Power source input terminal-   35 Power source of digital amplifying unit 13

1. A digital amplification device comprising: a digital amplifying unitincluding a differential input terminal for inputting an analog signaland a differential output terminal for outputting a digital signal, thedigital amplifying unit converting the analog signal to the digitalsignal, amplifying the power, and outputting the digital signal; alow-pass filter for integrating the digital signal; and a load outputterminal for outputting a signal from the low-pass filter, wherein thedigital amplification device further comprises a current feedbackcircuit for feeding back, to the differential input terminal, a signalrelated to a signal provided by transformer coupling with the low-passfilter.
 2. A digital amplification device according to claim 1, furthercomprising a voltage feedback circuit for feeding back voltage of adigital signal supplied from the differential output terminal to thedifferential input terminal.
 3. A digital amplification device accordingto claim 1, further comprising a transformer coupling type coil forbeing transformer-coupled to the low-pass filter, wherein a primary sideof the transformer coupling type coil is coupled so as to provide aninductance component of the low-pass filter, one terminal of a secondaryside of the transformer coupling type coil is grounded and the otherterminal is coupled to the current feedback circuit; and wherein anoutput signal of the low-pass filter is fed back to the differentialinput terminal.
 4. A digital amplification device according to claim 1,wherein the low-pass filter includes a toroidal double winding and acapacitor.
 5. A digital amplification device according to claim 1,further comprising: a second transformer coupling type coil having aprimary coil and a secondary coil formed of two windings, the windingsbeing wound in the mutually reverse directions; and a second currentfeedback circuit, wherein,the primary coil is disposed so as to providean inductance component of a line filter of a power source input andsupplies power source to the digital amplifying unit; wherein one sideterminals of the windings are grounded and the other side terminals ofthe windings are coupled to the second current feedback circuit; andwherein the second current feedback circuit feeds back a signal relatedto a signal from the windings to the differential input terminal.